Corrugated waveguide

ABSTRACT

The disclosed circular waveguide is provided on the inner wall surface with corrugated slots each having a width abruptly changed from a smaller value on that portion near to the axis of the waveguide to a larger value on the remaining portion of the slot. Also an electromagnetic horn is disclosed including such slots.

United States Patent 1191 Takeichi et a].

CORRUGATED WAVEGUIDE Inventors: Yoshihire Takeichi; Tsutomu Hashimoto;Furnio Takeda, all of Kamakura, Japan Mitsubishi 01111111 KabushikiK111151111, Tokyo, Japan Filed: Oct. 15, 1971 Appl. No.: 189,540

Assignee:

Foreign Application Priority Data Oct. 24, 1970 Japan 45/93689 US. Cl343/786, 333/33, 333/341 333/95 R, 333/98 R Int. Cl. H01q 13/02, H03117/38 Field of Search 333/95, 98, 98 M, 333/31 R, 34, 31 A, 33;343/772776, 786; 3l5/3.5, 3.6

References Cited UNITED STATES PATENTS 3/1971 Wilbur 333/31 X PrimaryExaminer-Rudolph V. Rolinec Assistant Examiner-Marvin Nussbaum itfornvE. F. Wenderoth, VQ C reedon et a].

[ ABSTRACT The disclosed circular waveguide is provided on the innerwall surface with corrugated slots each having a width abruptly changedfrom a smaller value on that portion near to the axis of the waveguideto a larger value on the remaining portion of the slot. Also anelectromagnetic horn is disclosed including such slots.

6 Claims, 7 Drawing Figures PAIENTEU ma 1 ma m A Du m R D: b B F m A R wW G O I 2 m s F H O CONDUCTANCE INVENTOR S d n a I HO CT 0 E M I mw 0% mS U 0 S Y T FUMIO TAKEDA,

G COQMPONENH ATTORNEY CORRUGATED WAVEGUIDE BACKGROUND OF THE INVENTIONThis invention relates to improvements in a corrugated waveguide.

The conventional type of corrugated waveguide has a plurality of annulardiscs or waveguide irises of the same dimension disposed atpredetermined equal intervals and perpendicularly to the axis thereof toform slots between the adjacent irises while defining central openingsaligned with one another. When a section of such a corrugated waveguidehaving a circular cross section is applied, for example, to anelectromagnetic born, the resulting directional pattern has been able tobe improved only over a frequency band of about one octave.

SUMMARY OF THE INVENTION Accordingly, it is an object of the inventionto provide an improved corrugated waveguide device having frequencycharacteristics maintained substantially uniform in a frequency bandwider than that previously obtained.

It is another object of the invention to provide an improvedelectromagnetic horn of the corrugated wave guide type havingdirectional characteristics substantially uniform over a frequency bandwider than that provided by the prior art practice.

The invention accomplishes these objects by the provision of acylindrical section of corrugated waveguide comprising a plurality ofannular discs disposed at predetermined equal intervals andperpendicularly to the axis of the waveguide to form slots therebetweenwhile defining central openings aligned with one another, wherein eachof the corrugated slots has a width which varies in the direction of thedepth thereof.

The width of the slot may preferably vary stepwise transversely of thedirection of the waveguide.

BRIEF DESCRIPTION OF THE DRAWING The invention will become more readilyapparent from the following detailed description taken in conjunctionwith the accompanying drawing in which:

FIG. la is a cross sectional view of a section of corrugated waveguideconstructed in accordance with the principles of the prior art;

FIG. lb is a longitudinal sectional view of the section shown in FIG. 1awith the longitudinal section taken along the line A-A of FIG. Ia;

FIGS. 2a and b are views similar to FIGS. la and b respectively butillustrating one form of the invention;

FIG. 3 is a longitudinal sectional view of an electromagnetic hornconstructed in accordance with the principles of the prior art;

FIG. 4 is a longitudinal sectional view of an electromagnetic hornembodying the principles of the invention; and

FIG. 5 is a schematic Smith chart useful in explaining the principles ofthe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS While the invention will bedescribed as being applied to a cylindrical corrugated waveguide havinga circular cross section it is to be understood that the same is equallyapplicable to cylindrical corrugated waveguides having a cross sectionwith the shape of any desired closed geometric figure such as arectangular cross section. The term cylindrical is also used in thebroadest sense. It is assumed that the corrugated waveguide of circularcross section has a hybrid mode or the HE,, mode and the EH,, modepropagated therethrough.

Referring now to the drawing and FIGS. 12: and b in particular, it isseen that the arrangement disclosed herein comprises a length of rightcircular cylindrical wave guide made up of a circular metallic tube 10,and a plurality of annular metallic discs 12 of the same 'dimensiondisposed at predetermined equal intervals and substantiallyperpendicularly to the axis of the tube 10 to form slots 14 therebetweenwith bottoms of the slots constituted by the inner wall surface of thetube 10.

It is assumed that the annular discs 12 frequently called waveguideirises have a pitch of P, an inside diameter of 2a and an outsidediameter of 2b as designated in FIG. 1b. The slots 14 shown in FIG. lbas having a width of d have a depth equal to (12-0). It is also assumedthat the waveguide illustrated has been constructed such that the pitchP is smaller than the wavelength it in the free space. Under the assumedconditions, the inner extremity of the iris 12, that is, the slot 14 ata point spaced from the axis of tube 10 by a distance a presents anadmittance Y in the hybrid mode expressed by the following equation:

where j unit of an imaginary number equal to UV T- s permittivity offree space 1. permeability of free space J, first order Bessel functionof a first kind Y, first order Bessel function of a second kind J firstorder differential of Bessel function J, Y, first order differential ofBessel function Y, K phase constant in free space equal to Zn/A It isnoted that the corresponding conductance is negligibly small so thatwhere B is the susceptance. For large values of K the equation (I) mayapproximate the equation v. 8, j(p/d) Vega-Lemme If the susceptance B isnull in the EH mode then the electromagnetic field has a component in aplane normal to the propagation axis high in intensity of the centralaxis of the waveguide and decreasing to a zero value at the distance afrom the central axis or on a circularly cylindrical surface formed ofthe inner edges of the irises 12.

In other words, when an electromagnetic horn is formed of a section of acorrugate'tlwaveguide such as above described, the same has adirectional pattern in the E plane coinciding with that in the H plane.Such an electromagnetic horn is shown in FIG. 3 wherein like referencenumerals designate the components corresponding to those illustrated inFIG. lb. FIG. 3 will be self-explanatory.

If B is infinitely large (or B. in the EH, mode it can be consideredthat the circularly cylindrical surface with the radius of a as abovedescribed is equivalently shortcircuited. This results in thecoincidence of the field distribution in the EH mode with that in the TEmode for a circular waveguide. Therefore for electromagnetic hornsutilizing the directional characteristics in the El-I mode such as shownin FIG. 3; the radius a is large so that the horns are superior indirection characteristics to conical horns operated in the TE mode in afrequency band holding the relationship higher in intensity thereonleading to the deterioration of the directional characteristics of theelectromagnetic horn.

Thus electromagnetic horns to which the conventional type of corrugatedwaveguides are applied such as shown in FIG. 3 are disadvantageous inthat a frequency band in which the directional characteristics can beexpected to be improved is restricted to a frequency band of about oneoctave expressed by the above relationship (3 The invention seeks toprovide wide band frequency characteristics for corrugated waveguidesand electromagnetic horns in the form of such waveguides.

In FIGS. 2a and b wherein like reference numerals designate thecomponents identical or similar to those shown in FIGS. la and b, thereis illustrated a section of corrugated waveguide of a circular crosssection constructed in accordance with the principles of the invention.The arrangement illustrated is different from that shown in FIG. 1 onlyin that in FIG. 2, the annular discs 12 or waveguide irises 12 each areprovided on one face with an annular land portion 16 radially extendingfrom the inner edge thereof to a predetermined radius of b, with theland portions 16 all on the same sides of the irises 12 in the exampleillustrated, on the left side as viewed in FIG. 2b. Thus the resultingslot 14 has an axial dimension of a width which varies in the directionof the depth thereof. More specifically, the' slot 14 has the widthvaried step wise from a predetermined fixed value d, in the region ofthe entrance thereof or for a S A 5 b to another predetermined fixedvalue of at greater than d in the region of the bottom thereof or for b,5 A b where A represents a radial distance from the axis of thewaveguide.

FIG. 4 shows an electromagnetic horn embodying the principles of theinvention. The arrangement illustrated comprises a throat portion 20 anda hom-shaped portion 22 connected thereto. A plurality of waveguideirises 14 with central land portions 16 similar to those shown in FIG.2b are disposed in both portions in the same manner as in thearrangement of FIG. 2b

excepting that those irises 14 disposed in the hornshaped portion 22follow in shape the latter to progressively increase in outside andinside diameters toward the open end of the horn-shape portion 22.

The arrangement of FIG. 2 will now be discussed in terms of theadmittance of the slot 14. As in the arrangement of FIG. 1 theconductance is also negligibly small and therefore it is required onlyto consider the susceptance. An admittance Y(b,) as viewed toward thebottom of the slot 14 at, a point at a radial distance of d from theaxis of the waveguide is given by the equation where Y is acharacteristic admittance of that portion of the slot having the widthd,, as will readily be understood from the deduction of the equation(2). The character B represents a corresponding susceptance. Similarly,the admittance Y(a) as viewed toward the bottom of the slot at a pointat a radial distance of a is expressed by the equation where Yrepresents a characteristic admittance of that portion of the slothaving the width d, and is according to the relationship.

Also B, is a corresponding susceptance.

The principles of the operation of the corrugated waveguide as shown inFIG. 2 will now be described with reference to FIG. 5 wherein there isschematically illustrated a Smith chart.

Conventional corrugated waveguides have a usable frequency band asdetermined by the equation (3). Assuming that the frequency band isdefined by frequencies f,, and f as the upper and lower limitsrespectively and that wavelengths in the free space are of A and A atthe frequencies of f and f respectively, the normalized B in theequation (I) occupies a position A or B shown in FIG. 5 at the frequencyf, or 1",, respectively. At any frequency f in the frequency banddefined by the frequerples f and f the normalized B is on a circular arcAEB.

It is now assumed that in the arrangement of FIG. 2, the slot has atotal depth (b a) equal to M14 and that portion thereof having the widthof d has a depth (b b greater than A 18 and less than A,j4. That is thefollowing relationships hold:

From the equation (4) and the inequality (8), it is apparent that thesusceptance B as viewed toward the bottom of the corrugated slot 14 at apoint radially spaced away from the axis of the waveguide by a distanceof b fulfils the inequality Thi normalized susceptance il /Y is on acircular arc ACB shown in FIG. 5. Assuming that it occupies a point C onthe circular arc (C l3, the susceptance B normalized by thecharacteristic inadmittance Y or B /Y,. will be moved from the point Ctoward the point A until it reaches a point D as will readily beapparent from the relationship (6).

This means that, with an angle COD represented by 6,, the normalizedsusceptance B /Y presented by the slot 14 as viewed at a point at aradial distance of a from the axis of the waveguide is turned from thepoint A toward the load through the angle 0, until it is located at apoint E shown in FIG. 5. Therefore it will be understood that at thelower limit of the frequency band or the frequency f the susceptance ofthe corrugated slot becomes null for conventional corrugated waveguidesand has a positive value for the present corrugated waveguides.

Considering the upper-limitf of the frequency band, it is apparent fromthe relationship A )\,,/2 that the normalized susceptance B /Y of thecorrugated slot as viewed toward the bottom thereof at a point at aradial distance of b, from the axis of the waveguide is turned from thepoint B through twice an angle a =LlOC to a point F as shown in FIG. 5.Normalizing the susceptance B, by the characteristic admittance Y ofthat portion of the slot having the width d causes the point F to bemoved toward the point A to reach a point G. Then the normalizedsusceptance B lY of the corrugated slot as viewed at a point at a radialdistance of a from the axis of the waveguide is turned thr ough an angleof 62 FOG toward the load until it is positioned at a point l-i shown inFIG. 5.

According to the invention, therefore, a normalized susceptance of theslot is on a circular arc EFH at any frequency in the frequency bandranging from f to f whereby the susceptances B s of the slots arepositive even at frequencies either lower than the f or higher than fThus the undesirable HE mode has an upper cut off frequency less than fAlso the upper limit of frequencies available for electromagnetic hornsoperated in the EH mode such as shown in FIG. 4 becomes higher than fwith the result that such electromagnetic horns have a frequency bandwider than the previously obtained.

By properly selecting the depth (b by) of that portion of the corrugatedslot 14, having the width d the susceptance of the slot can bemaintained substantially constant over a wider frequency band. As aresult, any electromagnetic horn including such slots is enabled toprovide a distribution of an electromagnetic field in the radiation modewhich is substantially constant over a wide frequency band, as comparedwith the prior art practice.

While the invention has been illustrated and described in conjunctionwith the application thereof to electromagnetic horns, it is to beunderstood that it is equally applicable to a variety of microwavedevices other than electromagnetic horns. For example, the invention maybe effectively applied to transformers for connecting a section of acircular waveguide to a section of a circular corrugated waveguide asdisclosed herein. This is because the admittance presented by the slotof the invention as viewed at a point having a radial distance of a fromthe axis of the waveguide can be selected to have any desired value byproperly changing the parameters d, d (b a) and/or (b b Specifically atransformer for connecting a section of a circular waveguide to asection of a circular corrugated waveguide may be formed of a section ofcircular corrugated waveguide designed and constructed in accordancewith principles of the invention such that corru gated slots on that endportion thereof adjacent to the end of the section of the circularwaveguide have admittances as high as possible. This is because theinner wall surface of the circular waveguide has an infinitely largeadmittance. Then as the slot gets nearer to the other end of thesection, the slot has an admittance approaching that admittance of thecorrugated slot of the circular corrugated waveguide to be connected.This results in a wide band matching.

While the invention has been illustrated and described in conjunctionwith a few preferred embodiments thereof, it is to be understood thatnumerous changes and modification may be resorted to without departingfrom the spirit and scope of the invention. For example, the inventionis equally applicable to square and rectangular waveguides.

What we claim is:

l. A section of waveguide comprising cylindrical wall means defining asection of a closed geometric figure, a plurality of annular metallicplates attached to the inner surface of said wall means at predeterminedequal intervals along and perpendicularly to the axis of the waveguideto define between them a plurality of slots with the inner surface ofsaid wall means defining the bottom of said slots, said plates havingcentral openings aligned with one another, each of said slots having awidth which varies in the direction of the depth thereof for causingsaid corrugated slots to have capacitive susceptances within a widefrequency band.

2. A section of waveguide as claimed in claim 1 having one end portionformed into a horn.

3. A section of waveguide as claimed in claim 1 wherein each of saidslots has a width which varies stepwise transversely of the axis of thewaveguide.

4. A section of waveguide as claimed in claim 3 having one end portionformed into a horn.

5. A section of waveguide as claimed in claim 1 wherein each of saidslots has a width which varies stepwise from a smaller value at thatportion thereof near the axis of the waveguide to a larger value at thatportion thereof near the inner wall surface of the waveguide.

6. A section of waveguide as claimed in claim 5 having one end portionformed into a horn.

k t 8 i it

1. A section of waveguide comprising cylindrical wall means defining asection of a closed geometric figure, a plurality of annular metallicplates attached to the inner surface of said wall means at predeterminedequal intervals along and perpendicularly to the axis of the waveguideto define between them a plurality of slots with the inner surface ofsaid wall means defining the bottom of said slots, said plates havingcentral openings aligned with one another, each of said slots having awidth which varies in the direction of the depth thereof for causingsaid corrugated slots to have capacitive susceptances within a widefrequency band.
 2. A section of waveguide as claimed in claim 1 havingone end portion formed into a horn.
 3. A section of waveguide as claimedin claim 1 wherein each of said slots has a width which varies stepwisetransversely of the axis of the waveguide.
 4. A section of waveguide asclaimed in claim 3 having one end portion formed into a horn.
 5. Asection of waveguide as claimed in claim 1 wherein each of said slotshas a widTh which varies stepwise from a smaller value at that portionthereof near the axis of the waveguide to a larger value at that portionthereof near the inner wall surface of the waveguide.
 6. A section ofwaveguide as claimed in claim 5 having one end portion formed into ahorn.